1. Field of the Invention
This invention relates to a device that measures the weak magnetic fields, both space and time dependent, which are generated by a source inside an object to be investigated. Such instruments are employed to detect magnetic fields elicited especially by the neural functions; the method is gaining gradually a more important role in medical diagnostics and research. In particular, it is possible to investigate the brain functions and disorders in a human being without touching the person or exposing him to electromagnetic radiation or radioactive tracers. In contrast to the widely used electroencephalogram (EEG), in which the electric potential distribution is measured on the surface of the scalp, the magnetoencephalogram suffers far less from distortions caused by inhomogeneities in the conductivity of the human tissue. Therefore, it is possible to locate source currents related to brain activity with a spatial and temporal resolution of a few millimeters and milliseconds. The method has been reviewed in detail, for example, in CRC Critical Reviews in Biomedical Engineering, volume 14 (1986), issue 2, pp. 93-126. Instruments used in MEG should be able to detect magnetic signals whose magnetic flux density is typically 100 fT or less. In addition, the measurement is to be performed simultaneously at several locations; the measurement of even more than one hundred magnetic signals from all over the head is necessary. The only sensor capable of detecting these minute signals is the so-called Superconducting Quantum Interference Device (SQUID) magnetometer. The operation of the device has been explained in detail in an article in Journal of Low Temperature Physics, volume 76 (1989), issue 5/6, pp. 287-386. The device requires a low operating temperature. Commonly, it is in a liquid helium bath inside a vacuum-insulated dewar vessel; the temperature is then 4.2K.
This invention focuses especially on the insert inside the dewar vessel; the SQUID magnetometer elements are attached to this insert. The insert must be stable enough and it must withstand tensions and changes in dimensions caused by the differential thermal contraction of various materials while cooling the device down to its operating temperature. At the same time, also the heat leak from room temperature to the helium bath must be minimized. The latter fact is of particular importance, because the device intended for measurements on a wide area necessarily has a large cross-section. The neck of the dewar needed for such a device must be wide; thus the heat leak by conduction, convection and radiation through the neck can substantially increase the helium boil-off rate and thus shorten the duty cycle between maintenance operations.
2. Description of the Related Art
A well-known prior-art structure of an insert (see, e.g. Review of Scientific Instruments, vol. 58 (1987), issue 11, pp. 2145-2156) consists of a fiber glass tube body, to which the metallic radiation shield baffles are attached in the neck region. In the lower end of the tube there is a holder to which the individual magnetometers or gradiometers are attached. All magnetometers or gradiometers can be on a same substrate as well, as has been presented in EP 111 827. The latter solution has, however, the drawback that all sensors must be in a common plane; thus it is not possible to adapt the device to the shape of the person to be investigated, as is possible when using individual sensor elements.
The electronic components which must be kept at liquid helium temperature are soldered on a printed circuit board attached to the tube body, and the wirings from the top plate connectors at room temperature are routed to liquid helium space inside copper-nickel tubes. The structure has, however, several drawbacks. The whole unit consists of one single component which is tedious and expensive to assemble and maintain. Especially in dewar vessels with wide-area necks a large space is left between the radiation baffles; the heat convects partially between two baffles because of the turbulence of the outflowing helium gas. Therefore, the temperature distribution tends to equalize in the vertical direction, increasing the boil-off rate unnecessarily. A neck plug made of foam plastic, attached to the tube body, has also been employed to circumvent the latter problem.
In another known solution (see DE 3 515 199) the body of the magnetometer has been divided into two parts. The so-called flux transformer coils of the magnetometer are in a fixed holder on the bottom of the dewar; this holder is then connected, via a multi-contact superconducting connector to a module that contains a group of SQUIDs, all inside a single element, a neck part and an electronics unit. The neck part contains all the necessary cabling and the thermal radiation shields; the electronics unit forms also the top plate for the dewar. In this construction, the holder with the flux transformers has been assembled already during the fabrication of the dewar vessel and can not be removed from the dewar via the neck made small to minimize the heat leak. As drawbacks one may mention that it is impossible to change or repair the flux transformers afterwards and that it is very difficult to make reliable multi-contact superconducting connectors. One may not that although the need for such connectors has existed, in different circumstances, already for twenty years no such connectors have been made in practice. The gradiometer holder has been fixed firmly and rigidly to the dewar bottom; in addition the module with the SQUID group, the neck plug, and the electronics unit is of rigid construction (column 4, lines 2-10). Therefore, when cooling the device down to its operating temperature, dangerously large tensions and stresses may be generated because of differential thermal contraction of various materials; these stresses may break the structure.
The radiation shields in the neck have in DE 3 515 199 been realized in an conventional way (see FIG. 4). Then, the convection problem between two successive baffles occurs. In addition, the module containing the SQUID group, the neck with cables and the electronics unit form a single piece which is difficult to disassemble for eventual maintenance. Furthermore, the referred publications do not present any solution to the problem that arises when there are very many channels, on the order of 100: the cabling in the neck easily dominates the heat leak. This problem can be circumvented by choosing the conductor material to be very resistive, but then, the noise of the sensors will increase in intolerably if conventional read-out methods are used.
The DE 3 515 199 also discusses the possibility of making the cabling by patterning a flexible printed circuit board. This solution, although very elegant and efficient in the manufacturing point of view, has a drawback: the conductor materials on standard printed circuit boards have high conductance and a resistive material to reduce heat leak cannot be chosen freely. It is also impossible to twist the wires to increase the immunity to interference magnetic fields and cross-talk between the channels.
DE 3 515 237 deals with a similar array of gradiometers as DE 3 515 199; in particular, the SQUID group, which has been integrated as a single module on a common substrate, and its internal structure are discussed. Specific attention is paid to the arrangement of wirings via groundplanes and to the magnetic shielding of the SQUIDs by means of superconducting loops and groundplanes. In this aforementioned application, the SQUID chip has been attached to a printed circuit board, onto which the conductors have been patterned. Also, the electronics unit forms the top plate of the dewar vessel (see FIG. 1). Furthermore, the assembly of flux transformers and the SQUID group have been connected in a way that can be easily disconnected, for example via a multi-contact superconducting connector. The drawbacks of this construction are mainly the same as in the first referred publication. In particular, the problems are enhanced when the number of channels in large, on the order of one hundred of more. A complete modularity that is necessary for reliability, easy testability and ease of manufacture is not reached, completely reliable superconducting multicontact connectors are difficult if not impossible to manufacture, and if one SQUID fails, the complete group of SQUIDs has to be changed. In addition, the heat leak problem has not been solved by the conventional wiring and radiation shield structures utilized in the referred publication. Neither has the differential thermal contraction been taken into account.
EP 361 137 discusses a magnetometer that can be positioned in an unconventional way, upside down so that the gradiometer coils are topmost. Liquid helium is inside a separate vessel inside the dewar; the cold helium gas boiling off this inner vessel is led, via tubes, to cool the gradiometer coils. The SQUID group is inside a separate unit in the liquid helium container. The neck of the dewar is substantially narrower than the gradiometer part; thus, most parts have to be assembled in place when the dewar is being made. To prevent counterflow of warm helium gas from outside to inside, a special constriction with relief flaps has been constructed in the neck. The basic idea of this device is significantly different from what is aimed at in our invention; thus, for example the neck is of completely different structure, and the whole structure is not modular in the sense of what would be desired. When the number of channels approaches one hundred, it is reasonable to assume that an even and reliable cooling of the gradiometer coils cannot be accomplished with a moderate liquid helium consumption. To maintain such an instrument with many fixed parts inside the dewar would also be difficult.
In a conventional read-out of SQUIDs the heat conduction through the cables in the neck can be dominating, when the number of channels is large. To minimize noise, the resistance of the wires connected to the SQUIDs has traditionally been kept as small as possible. Because of the Wiedemann-Franz' law, a large heat leak follows necessarily. For example, a resistance of 1 .OMEGA. in a wire leading from room temperature to liquid helium gives rise to a thermal load of 2 mW, if the cooling due to the outflowing helium gas is neglected and the change of resistivity of the wire as a function of temperature is negligible. In a hundred-channel instrument, where six conductors per channel are required, this means a boil-off rate of 1. 7 liters/hour; this is approximately ten times too high for a practical device.